Confinement effect of carbon nanotubes on the chain mobility of conjugated polymer poly(9,9-dioctylfluorenyl-2,7-diyl)

Abstract

The conjugated polymer polyflourene has been well studied for its strong blue light emission ability and high quantum efficiency behavior. It has wide applications for light emitting diodes, sensors as well as photo-detectors. Therein the <inline-formula><tex-math id="Z-20181210033236-1">\begin{document}$ \beta $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20182008lilingdong_Z-20181210033236-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20182008lilingdong_Z-20181210033236-1.png"/></alternatives></inline-formula> conformation of PFO crystals is more attractive due to its longer conjugation length, higher carrier mobility and better luminous efficiency. Therefore it is great essential to control the formation and stability of <inline-formula><tex-math id="Z-20181210033236-2">\begin{document}$ \beta $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20182008lilingdong_Z-20181210033236-2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20182008lilingdong_Z-20181210033236-2.png"/></alternatives></inline-formula> conformation of PFO crystals to develop new kind of photo-electronic devices. As is known, polymeric materials confined in a nanometer-sized space often exhibit unique properties compared with their bulk state, such as abnormal chain mobility, molecular assembly and phase transition behavior. These factors are of great significance to develop new kind of material and applications. Generally the confined condition includes quantum dot (zero-dimensional, 0D), nanowire or nanotube (1D), ultrathin film (2D) and nanoparticle (3D). In this paper, we design a unique 1D nanoconfined environment based on vertically aligned carbon nanotube (CNT) array structure. An ultra-high CNT density is achieved through a solvent-induced contraction process. The adjacent narrow carbon nanotube gap thus forms a quasi-1 confined nano-space with the tunable size ranging from 5 to 50 nm. Then we infiltrate the conjugated polymer poly(9,9-dioctylfluorene-2,7-diyl) (PFO) into those nano-gaps of carbon nanotube arrays through a solvent evaporation method to obtain the PFO infilled CNT array composite film. It is found that the chain mobility of PFO molecules in such a 1D nano-confined space of carbon nanotubes is significantly suppressed compared with the scenario of the spin-coated PFO film. The transition speed between different crystal forms of PFO declines greatly, which meanwhile improves the thermal stability of the <inline-formula><tex-math id="Z-20181210033236-3">\begin{document}$ \beta $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20182008lilingdong_Z-20181210033236-3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20182008lilingdong_Z-20181210033236-3.png"/></alternatives></inline-formula> conformation of PFO crystal. Additionally, the aligned carbon nanotubes have great effects on the orientation and distribution of PFO chains. The PFO crystals are confirmed to grow preferentially along the longitudinal direction of CNT array, which is potential to grow PFO crystals with high quality and excellent performance. Thus, such a PFO/CNT array composite film can have great potential to prepare PFO photovoltaic devices with excellent luminescent properties and high stability in the future.